1. Field of the Invention
The present invention relates to an optical disc apparatus using an optical disc and light, for reproducing data recorded on the optical disc.
2. Related Art
These years, there has been used, in general, a semiconductor laser, specifically a semiconductor laser composed of a direct semiconductor, as a light source for light with which data is read (reproduced) from an optical disc recording thereon the data. That is, all semiconductor lasers which have been conventionally so-called are direct semiconductor lasers using a direct semiconductor as an active layer.
The semiconductor laser composed of a direct semiconductor is highly monochromatic, and is highly and intrinsically active to produce light having a single emission wavelength, and with no specific contrivement, the light is quantized with a length of a resonator, and tends to oscillate with a certain single wavelength. Accordingly, the oscillation spectra exhibit a very sharp intensity distribution as a needle with respect to a wavelength 53, as shown in FIG. 1 (Refer to a direction transmission type 2), and its typical half-value width is in general, less than 0.01 nm.
However, when intensity modulation is made to this laser beam in order to record or transmit data, the output light intensity does not smoothly change in accordance with a current value due to mode-mopping or the like on the way of variation of the output optical intensity, so as to cause a problem of producing the so-called laser noise. Further, although the semiconductor has exhibited an extremely high efficiency of light emission so as to be used as a light source for an optical disc apparatus of the light having excessively high interference and monochromatism is used as a light source for reproduction of an optical disc unit, partially reflected light from an optical component such as a lens, a reflector, a prism or a beam splitter interferes with a reproduction light beam which is inherently detected so as to produce optical interference noise which has been known as scoop-noise, resulting in detrimental affect upon the reproduction of signals. Such optical interference noise is caused by such a fact that the monochromatism of the laser source is too high, and accordingly, sharp optical interference is caused.
In order to prevent occurrence of such optical interference noise, there have been raised various proposals so as to contrive the structure of a laser element and a drive method thereof. This problem has been also discussed not only in the optical disc field but in the optical communication field, as stated in Japanese Laid-Open Patent No. H8-330680. In general, in order to solve the above-mentioned problem, there have been used a semiconductor laser of multimode oscillation type formed therein with a gain waveguide structure by doping, as disclosed in Japanese Laid-Open Patent No. H11-87831, a semiconductor laser having such a configuration that a saturable absorbing layer is formed so as to carry out self-modulation oscillation as disclosed in Japanese Laid-Open Patent No. 2000-68610, and a technique of high-frequency convolution in which a usual laser having a single emission wavelength is sued, and a laser exciting current is finely turned on and off by a high frequency wave so that the continuity of oscillation is cut off, that is, there have been taken the contrivement of preventing oscillation with a single wavelength. In particular, the technique of high frequency convolution is effective since it is capable of lowering laser noise, and accordingly, it has been prosperously used in the field of optical discs or optical communication.
However, the above-mentioned countermeasures have not been completely sure, the angular distribution of light emission becomes wider in a laser noise reduction method using self-oscillation, and it is likely to cause a problem of astigmatism at the lens. Further, even in the case of using the multi-mode oscillation type semiconductor laser, as disclosed in Japanese Laid-Open Patent No. H11-87831, there cannot be completely prevented such a mode hopping-like change in light volume that the emission wavelength intensity randomly varies while causing interference between bright lines, which mainly causes in particular optical noise during reproduction. Further, the high-frequency wave convolution disclosed in the Japanese Laid-Open Patent No. H11-87831 requires to arrange a high-frequency oscillation circuit in the vicinity of the laser, and accordingly, in a configuration in which both semiconductor laser and head should be moved, it is required to arrange the oscillation circuit on the movable head, and accordingly it mainly causes the costs to increase. Further, as the optical head becomes small-sized, the time by which the light beam goes away and comes back through the optical path becomes shorter, the frequency of the convolution wave should be increased further, and accordingly, it is required to take a countermeasure for preventing electromagnetic radiation. Thus, it mainly causes the costs to increase.
Further, even in any of the methods, as the recording density of an optical disc becomes higher, a margin for laser noise or optical interference noise becomes severe, thereby it is required to reduce the noise.